At suggested dosing levels, inverts and all plants (in my experience) are fine with it.
If you use it at higher doses to kill or impede algae (1mL/5g or more daily), your bryophytes (mosses, hydrocotyle, pelia) and vals may start to suffer for it.
I tried using it because I was (and still am) intimidated by the whole DIY CO2 thing. I am totally not good when it comes to anything involving chemistry! I was hoping Excel would help the plants grow and give them the necessary carbon without having to mess w/the CO2 system. Even at regular dosing levels Excel clouded up my tank water every time - and it stayed cloudy until I did a water change. I finally pitched it, because I didn't see that the plants were growing any better with it anyway.